1. Field
The present specification generally relates to optical fibers and, more specifically, to polarization maintaining optical fibers.
2. Technical Background
Polarization-Maintaining (PM) optical fibers are a class of specialty optical fibers that may be used in a variety of applications including coherent communication systems, navigational controls (gyros), fiber-optic sensors, and, most-recently, in high-brightness fiber lasers.
Conventional PM optical fibers are made by inserting stress-applying-parts adjacent to and external to the core (i.e., the light carrying portion of the optical fiber) in order to induce a non-symmetrical stress field around the core. The stress-applying-parts (SAPs) are generally formed from boron-doped silica glass. The SAPs can have various shapes depending on the methods used for making the fiber. For example, Panda-type PM optical fibers are formed by mechanically drilling a pair of holes around the core of the optical fiber preform and inserting SAPs in the holes prior to drawing the preform into a fiber. The birefringence generated in the PM fibers is due to the anisotropic thermal-stress caused by the mismatch in the coefficient of thermal expansion between the SAPs and the surrounding cladding.
Due to the high absorption of boron doped SAPs, the SAPs must be positioned some distance away from the fiber core in order to avoid high fiber attenuation. Very high boron doping levels (>20% by weight) are generally needed to produce sufficient birefringence in the core. In addition, boron doping in the glass changes the viscosity of the glass significantly, making the glass much softer than the pure silica cladding. As a result, PM fibers with boron doped stress rods are difficult to make and expensive. In the case of multi-core PM fiber or PM fibers with a low cladding diameter, there may be insufficient space in the cladding portion of the optical fiber to accommodate the SAPs.
Accordingly, a need exists for alternative designs for polarization maintaining optical fibers.